When you encounter wild Pokemon, there is 'Escape' icon at the top left corner of the screen. By tapping the icon, you can run away from them. The escape function may be used for saving Pokeballs.
After escaping from the wild Pokemon, the Pokemon stays on the map. By tapping it, you can try to capture again.
If the Pokemon runs away from you, it does not appear on the map again.
After escaping wild Pokemon, you can try to capture them again as mentioned above. If you do so, the Pokemon's CP do not change. They appears having same CP.
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